Magnetite scavenging and the buoyancy of bubbles in magmas. Part 1: Discovery of a pre-eruptive bubble in Bishop rhyolite

Little is known about the presence, distribution and size of bubbles in rhyolitic magmas prior to eruption. Using X-ray tomography to study pumice from early-erupted Bishop rhyolite, we discovered a large vesicle with abundant magnetite crystals attached to its walls. Attachment of magnetite crystals to bubble walls under pre-eruptive conditions can explain the cluster of magnetite crystals as a result of bubbles rising and collecting magnetite crystals. Alternatively, bubbles may have nucleated on magnetite crystals and then coalesced to form one large bubble with multiple magnetite crystals attached to it. We argue that the clusters of magnetite crystals could not have formed during or after eruptive decompression, and conclude that this vesicle corresponds to a bubble present prior to eruptive decompression. The inferred presence of pre-eruptive bubbles in the Bishop magma confirms the interpretation that the magma was volatile-saturated prior to eruption. The pre-eruptive size of this bubble is estimated based on three independent approaches: (1) the current size of the vesicle, (2) the total cross-sectional area of the magnetite crystals, and (3) the bubble size required for the aggregate to be neutrally buoyant. These approaches suggest a pre-eruptive bubble 300-850 mu m in diameter, with a preferred value of 600-750 mu m.